This invention relates broadly to the field of polymer chemistry, and more particularly to unsaturated ethylene polymers having active carbon-carbon double bonds in the polymer chains and to processes for preparing and for using the same.
The field of polymer chemistry is ever changing, with the discovery of new compositions and processes for particular uses or for achieving specified results in a new or improved manner. Polymers, generally, are long chains of atoms having high molecular weights and formed by the linking of simpler molecules known as monomers through sharing of electrons. This electron sharing is termed "covalent bonding" and is characteristic of certain unsaturated hydrocarbons exhibiting a carbon-carbon double bond in their chemical structure. One such family of hydrocarbons is the alkenes of which ethylene, C.sub.2 H.sub.4, is the simplest member.
Polymerization is the process of joining together these many small molecules to make very large molecules. The particular kind of polymerization undergone may be addition polymerization, in which many molecules of monomer are simply added together, or condensation polymerization, in which monomer molecules join by the elimination of small molecules, usually water. Proteins and polysaccharides are examples of condensation polymers. Nylon, like the proteins, is a polyamide formed by the condensation of a diamine (such as hexamethylenediamine, H.sub.2 N(CH.sub.2).sub.6 NH.sub.2) and a dicarboxylic acid (such as adipic acid, HOOC(CH.sub.2).sub.4 COOH).
Addition polymerization, on the other hand, is thought to proceed by either a free-radical or an ionic mechanism. Both require the presence of at least a small amount of an initiator or catalyst. The free-radical type uses initiators such as peroxides which break down to form free radicals which add to monomer molecules, and in so doing generate other free radicals which eventuate in the long chain molecule. The ionic type causes polymerization through the involvement and breakdown of intermediate ions such as positive ions (cations), when the catalyst is an acid, or negative ions (anions), when the catalyst is a base. This newer ionic polymerization is considered by some to be particularly beneficial in view of its often milder reaction conditions and the stereochemical control it provides.
When ethylene gas, mentioned above, is placed under extreme heat and pressure in the presence of oxygen and under the influence of a catalyst or initiator, its carbon-carbon double bond will open in a polymerization reaction. The compound obtained is called polyethylene, and consists of a long chain of molecules which may exhibit a wide range of molecular weights (between about 1,000 and about 100,000 or higher). It has a wax-like consistency and properties that vary according to both molecular weight and type, e.g., low-density or branched, and high-density or linear. The linear type is typically more crystalline, more heat-resistant, and stiffer than the low-density or conventional type. Both have low water absorption, excellent electrical resistance, high resistance to most solvents and chemicals, and are tasteless and odorless. Polyethylene is well suited to many commercial applications, particularly where only moderate to low heat exposure is encountered. It is most familiar as the plastic material used for packaging films and various other flexible molded plastics.
As the uses for ethylene polymers have grown over the years, so also have the possible approaches to tailoring specific properties by manufacture or formulation to better suit the same. Physical properties such as melting point ranges, specific gravity, dielectrics, stress and crack resistance, fatigue, flexibility, toughness, and crystallinity are often the targets of this tailoring process. One common approach is through the presence of side branches in the polyethylene chain which can cause variations in properties such as density, hardness, flexibility, melt viscosity, transparency, and others. Chain branching, however, is a characteristic of the polymerization process and is not easily changed after the long chain molecules are formed.
Compounding and copolymerization are two other common methods. Compounding involves the addition of certain additives to polyethylene resins to arrive at desired properties. Common examples include stabilizers (such as antioxidents and carbon black), slip and antiblock agents, antistatic agents, and pigments. A modifier such as polyisobutylene or butyl rubber or others may also be used to compound the resin. Copolymerization, on the other hand, involves the mixing of two or more unsaturated monomers which are then allowed to polymerize together. The resulting polymer contains units of both monomers which may exhibit distributions ranging from complete randomness to strick alternation along the polymer chain. In this regard, much work has been done with copolymerizing ethylene and other hydrocarbons such as 1-butene, styrene, acrylic, acrylonitrile, vinyls and other substituted dienes.
Physical blending of different polymers, in the melt phase and otherwise, is yet another procedure for attempting to achieve a desirable balance of physical or chemical properties for a particular application. Problems of achieving homogeneous blends are often encountered, however, which cause added problems in view of the many fabrication techniques employed in the industry, such as injection, blow, and rotational molding, vacuum forming, and others.
Cross-linking of polymer chains is often used in tailoring properties of thermoplastic polymers such as polyethylene for high-temperature applications, for clarity of high-density materials, and for other uses. By its definition, this method involves the polymer molecules becoming linked together to produce a three-dimensional structure. The resulting product is no longer thermoplastic, but is thermoset and is useful for applications such as wire and cable coatings and many others. Cross-linking proceeds by two known mechanisms. The first is by chemical reaction and requires high temperatures and high pressures (often inert) and the presence of cross-linking agents such as organic peroxides, diperoxides and hydroperoxides, azo-compounds, sulfur or sulfur compounds, and many others. The second is by irradiation in which the activation energy required to cross-link the polymer chains is typically applied through high-energy beta or gamma radiation. Both methods are complex and often costly, the first requiring harsh conditions and ancillary agents which may become entrapped within the cross-linked structure as unwanted impurities while the second requires costly equipment and extreme care during use.
Moreover, with all the above methods except copolymerization, which proceeds from the monomer stage, problems are encountered in obtaining the desired properties by changing the ethylene or other polymers once the long chain molecules are formed. Polymers generally are thought of as not highly reactive compounds. Polyethylene, in particular, is considered very inert as being a saturated molecule having no unoccupied bonding sites. It is for this reason that drastic measures (such as irradiation or chemical attack using outside agents) are required to alter its structure once formed. Physical blending of formed ethylene or other polymers can achieve some change in properties, but is no solution to the problem.
The need exists for new ethylene and other polymers more readily susceptible to structural change to permit simple and expedient tailoring of physical properties. The need also exists for new and improved processes for preparing and for using the same. Applicants' present invention addresses these needs.